Bilirubin sensor

ABSTRACT

A bilirubin sensor has a working electrode with a first chemical matrix disposed thereon that contains a binder, a substrate electrode with a second chemical matrix dispose thereon that contains a binder and a chemical agent that consumes bilirubin, a reference electrode, a sample chamber containing the working electrode, the substrate electrode and the reference electrode, and a method of measuring bilirubin in a body fluid.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electrochemical sensors thatcan be used for quantification of a specific component or analyte in aliquid sample. Particularly, the present invention relates to anelectrochemical-based sensor for measuring bilirubin in biologicalfluids.

2. Description of the Prior Art

Bilirubin is the main bile pigment that is formed from the breakdown ofheme from red blood cells. The broken-down heme travels to the liver,where it is secreted into the bile by the liver. It is estimated thatapproximately 200 to 300 milligrams of bilirubin and its derivatives areformed each day in a normal human adult by degradation of hemoglobinwithin the liver, spleen, and bone marrow. Normally, a small amount ofbilirubin circulates in the blood. Serum bilirubin is considered a truetest of liver function as it reflects the liver's ability to take up,process, and secrete bilirubin into the bile.

Two fractions of bilirubin are present in blood serum. One is free ornon-conjugated bilirubin and the other conjugated bilirubin since it hasbecome conjugated with glucuronic acid and rendered water-soluble. Thediagnostic significance of bilirubin is well established. An excessiveamount of bilirubin within the human body, referred to as jaundice, isrecognized as evidence of a variety of disease conditions, particularlydiseases of the liver. In an adult, bilirubin could be excreted from thebody by way of the bile fluids. In a fetus, bilirubin can easily crossthe placenta and be removed through the mother's liver. Newborn infantsand especially prematurel infants, however, are particularly susceptibleto hyperbilirubinemia. Hyperbilirubinemia is defined as an abnormallyhigh level of bilirubin in the blood, manifested by jaundice, anorexia,and malaise, occurring in association with liver disease and certainhemolytic anemias. Because an infant's liver is not fully functioningfor the first few weeks after birth, bilirubin may accumulate abovenormal adult levels, which “stains” infant's skin leading to jaundice.Clinically, untreated jaundice in neonates can lead to impaired learningability, mental retardation, cerebral palsy, seizures, deafness, or evendeath.

There are several methods used to determine bilirubin concentration. Theleading method is the so-called diazo method, which measures thepurplish-red color of azobilirubin formed by the reaction of bilirubinwith a diazo compound. There are numerous versions of the diazo method,differing in the reaction conditions and reagent composition. The methodaccording to Jendrassik and Gróf has been recommended by the NationalCommittee for Clinical Laboratory Standard (NCCLS) as the procedure ofchoice for total bilirubin measurement. There are, however,disadvantages associated with this method. For example, the reagentsolutions for the Jendrassik/Gróf method can be used for only about 5days after their preparation, and the measured values are affected byL-ascorbic acid and hemoglobin, which are present in a sample.

The total bilirubin level in bodily fluids has been measured by directspectrophotometric estimation at 454 nm with correction at 540 nm. Thisdirect spectrophotometric measurement has been improved by makingobservations before and after the destruction of bilirubin with theenzyme bilirubin oxidase. O'Leary et.al (Ann. Clin. Biochem.,30:175-179, (1993)) describes a two-step method based on the measurementof absorbance of bilirubin itself at about 480 nm, followed bymeasurement of absorbance at 480 nm after destruction of the bilirubinwith ferricyanide. This method is limited due to the use of the 480 nmwavelengths of detection. At this wavelength, highly lipemic orhemolyzed sera may cause the absorbance, and a false result may beproduced.

U.S. Pat. No. 6,326,208 (2001, Denney) discloses an assay for measuringtotal and direct bilirubin colorimetrically that includes the use of afirst or a second reagent, respectively. For total biliruben, the firstreagent includes a solvent in which the bilirubin and the proteins aresoluble, an acid, and a promoter for speeding up the oxidation of thebilirubin and the formation of a chromophore that is detectable in thenear-infrared region of the spectrum. Useful promoters are nitrite ion,cupric ion and ferricyanide ion. For direct bilirubin, the secondreagent includes an aqueous component containing an acid for dissolvingboth the serum or plasma proteins and the conjugated bilirubin of thesample, and an oxidant for promoting the oxidation of the conjugatedbilirubin while promoting very little oxidation of unconjugatedbilirubin. The oxidant should have a low single electrode potentialbetween 0.34 and 0.78 volts. Useful oxidants are cupric ion, ferric ionand ferricyanide ion.

U.S. Pat. No. 5,563,072 (1996, Tokuda et al.) discloses a method ofmeasuring direct bilirubin calorimetrically that includes usingbilirubin oxidase or a chemical oxidizing agent of vanadic ions,manganese (III) ions, ferricyanide ions or copper ions. Another methodincludes additionally using a reaction inhibitor that is a water-solublehigh polymer or a nonionic surfactant having a HLB value of 15 or more.

Yet another method involves the use of a dry film for determination ofserum bilirubin, which is based on the observation that bilirubininteracts with certain cationic polymers (mordants) giving a uniquespectral change. A transparent support is coated with a bufferedgelatin-mordant layer and an isotropically porous polymer spreadinglayer. The spreading layer contains all of the necessary components(diazonium salt, accelerator, surfactant) for the quantification ofbilirubin. The buffered gelatin-mordant layer maintains the pH of thedry film, minimizes the dependence of spot size on proteinconcentration, and increases the sensitivity of the dry film. The dryfilm, however, is only stable for 4 weeks at room temperature and cannotbe used for whole blood analysis.

Therefore, what is needed is a bilirubin sensor that is usable withwhole blood. What is also needed is a bilirubin sensor that is capableof performing an accurate and sensitive analysis of the concentration ofbilirubin in a small volume of biological sample. What is further neededis a bilirubin sensor that is disposable.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a bilirubin sensorcapable of measuring the bilirubin concentration in a small volume ofbodily fluid. It is another object of the present invention to provide abilirubin sensor that is usable without routine maintenance. It is afurther object for the present invention to provide a bilirubin sensorthat measures bilirubin in a fluid sample electrochemically. It is stillanother object of the present invention to provide a bilirubin sensorthat includes an interference-correcting electrode to minimize theinterference caused by the presence of other oxidizable components inthe sample fluid. It is yet another object of the present invention toprovide a disposable bilirubin sensor. It is still a further object ofthe present invention to provide a disposable bilirubin sensor that iscapable of measuring the bilirubin concentration in biological fluidincluding blood obtained by lancing the finger of the user. It isanother object of the present invention to provide a disposablebilirubin sensor for measuring bilirubin concentration in a small volumeof whole blood.

The present invention achieves these and other objectives by providing adisposable bilirubin sensor having a working electrode, a substrateelectrode and a reference electrode disposed within a sample chamberconfigured for receiving a liquid sample. The working electrode includesan electrical conductor with a first reagent matrix disposed thereonthat contains a binder. The substrate electrode includes an electricalconductor with a second reagent matrix disposed thereon that contains abinder and a chemical agent that consumes bilirubin to prevent thebilirubin from being oxidized at the surface of the substrate electrodeby a voltage that is applied to the bilirubin sensor. The first reagentmatrix and the second reagent matrix can also each contain a whole bloodfiltering component. The reference electrode includes an electricalconductor and a reference reagent matrix disposed thereon. The referencereagent matrix may be any reference material including, but not limitedto, a binder, silver-silver chloride or other reference electrodematerials.

Bilirubin is a reductant that is easily oxidized at an electrode surfaceif a sufficient positive potential is applied at the electrode.Bilirubin is also easily oxidized by a chemically oxidizing reagent suchas ferricyanide compounds, cupric compounds, nitrite compounds ,Ruthenium compounds, osmium compounds, and the like. When using redoxmediators to oxidize bilirubin, the redox mediator changes from itsoxidation form to its reduction form (for example from Fe(CN)₆ ³⁻toFe(CN)₆ ⁴⁻for ferricyanide compounds). The reduced redox mediator (suchas Fe(CN)₆ ⁴⁻) can be re-oxidized electrochemically to its oxidationform (such as Fe(CN)₆ ³⁻) at the electrode surface when a positivepotential is applied to the electrodes. The resulting current is relatedto the bilirubin concentration.

Unfortunately, other species in a sample fluid such as a body fluid arealso capable of undergoing a similar reaction and causing a falsedetermination of bilirubin concentration. These other oxidizable speciesinclude ascorbic acid, uric acid, acetaminophen, and the like. Thepresent invention provides an apparatus and a method for a more accurateelectrochemical measurement of bilirubin concentration in biologicalfluid samples in the presence of these other interfering species. Animportant feature of the present invention that accomplishes this moreaccurate measurement is the substrate electrode. The substrate electrodeis loaded with a chemical agent that consumes bilirubin to prevent itfrom being oxidized by the chemically oxidizing reagent. The chemicalagent is preferably a bilirubin sensitive enzyme and, more preferably,bilirubin oxidase.

The resulting current from the working electrode is related to all thereductants in the sample fluid and the current from the substrateelectrode is related to all the reductants in the sample fluid exceptbilirubin. The differential current between the working electrode andthe substrate electrode is related to the concentration of bilirubin inthe sample fluid. Therefore, use of a working electrode and a substrateelectrode provides a more accurate measurement and correlation to theconcentration of bilirubin. In addition, because the current arisingfrom the reductants except bilirubin is measured, any variation in theconcentration of these oxidizable species from sample to sample isnullified and, thus, provides a more accurate measurement of bilirubinin the sample fluid.

In addition to the concentration of bilirubin, the differential currentis also influenced by hematocrit when the fluid samples are bloodsamples. The effect of hematocrit can be eliminated by measuring theimpedance (Imp) or resistance (R) of the blood samples. For purposes ofthe following equations, impedance and resistance is usedinterchangeably since the measurement value for the impedance orresistance is Ohms. Hematocrit effect can be corrected and eliminatedusing the following equations:H=(a ₁)(R ²)+(a ₂)(R)+a₃   (1)where R is the resistance value measured in Ohms, H is hematocrit value,and a₁, a₂ and a₃ are constants.

The hematocrit value is then used to mathematically correct thebilirubin concentration measured using the above described sensor. Thefollowing equation represents the calculation preformed using thecalculated hematocrit level from equation (1):C _(corr)=(C _(mea))((b)(H ²)+(c)(H)+d)   (2)

-   -   where C_(corr) is the corrected analytical concentration        -   C_(mea) is the measured analytical concentration        -   b, c, and d are constants        -   H is the hematocrit level calculated from equation (1)

The constant values “a” through “d” above are especially determined anddependent on several factors such as the arrangement of the electrodes,the formulations of the chemical reagents, the surface area of theelectrodes, and the ratio of the surface areas of the electrodes. Oncethese factors are set, numerical analysis is performed on the test dataobtained from predefined sample fluid standards using a plurality ofsensors having these preset factors in common. The correction can alsobe done by using the ohmic value without converting the ohmic value tohematocrit by simply substituting equation (1) into equation (2) andcreating an algorithm that uses the ohmic value to correct for theeffect of hematocrit in the sample fluid.

All of the advantages of the present invention will be made clearer uponreview of the detailed description, drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of the present inventionshowing the test strip.

FIG. 2 is an exploded view of the embodiment in FIG. 1 showing the fourcomponent layers of the test strip.

FIG. 3 is a perspective view of another embodiment of the presentinvention showing the test strip.

FIG. 4 is an exploded view of the embodiment in FIG. 3 showing the threecomponent layers of the test strip.

FIG. 5 is a perspective view of another embodiment of the presentinvention.

FIG. 6 illustrates the correlation between the current response of thebilirubin sensor of the present invention to bilirubin concentrationusing ferricyanide and bilirubin oxidase in the chemical matrix.

FIG. 7 illustrates the precision of the bilirubin sensor of the presentinvention at two different bilirubin concentrations.

FIG. 8 illustrates the correlation between the current response of thebilirubin sensor of the present invention to bilirubin concentrationusing hexaamineruthenium (III) chloride and bilirubin oxidase in thechemical matrix.

FIG. 9 illustrates the correlation between the current response of thebilirubin sensor of the present invention to bilirubin concentrationusing chemical matrix that does not include a chemically oxidizingreagent/redox mediator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment(s) of the present invention is illustrated inFIGS. 1-9. FIG. 1 illustrates one embodiment of the present invention.The bilirubin sensor 10 has a laminated body 12, a fluid sampling end14, an electrical contact end 16, and a vent opening 52. Fluid samplingend 14 includes a sample chamber 17 between a sample inlet 18 and ventopening 52. Electrical contact end 16 has three discrete conductivecontacts 16 a, 16 b and 16 c.

Turning now to FIG. 2, laminated body 12 includes a base layer 20, areagent holding layer 30, a channel forming layer 40, and a cover 50.All layers of laminated body 12 are made of a dielectric material,preferably plastic. Examples of a preferred dielectric material arepolyvinyl chloride, polycarbonate, polysulfone, nylon, polyurethane,cellulose nitrate, cellulose propionate, cellulose acetate, celluloseacetate butyrate, polyester, polyimide, polypropylene, polyethylene andpolystyrene.

Base layer 20 has a conductive layer 21 on which is delineated threeconductive paths 22, 24 and 26. The conductive paths 22, 24, 26 may beformed by scribing or scoring conductive layer 21. In the alternative,base layer 20 may be a dielectric material on which conductive paths 22,24, 26 are silk screened.

Scribing or scoring of conductive layer 21 may be done by mechanicallyscribing the conductive layer 21 sufficiently to create the threeindependent conductive paths 22, 24, 26. The preferred scribing orscoring method of the present invention is done by using a carbondioxide laser, a YAG laser or an eximer laser. Conductive layer 21 maybe made of any electrically conductive material such as, for example,gold, tin oxide/gold, palladium, other noble metals or their oxides, orcarbon film compositions. The preferred electrically conductive materialis gold or tin oxide/gold. An additional scoring line 28 (enlarged andnot to scale; for illustrative purposes only) may be made along theouter edge of base layer 20 in order to avoid possible static problemsthat could give rise to a noisy signal. It should be understood,however, that scoring line 28 is not necessary to the functionality ofsensor 10. Conductive layer 21 may be made of any electricallyconductive material such as, for example, gold, tin oxide/gold,palladium, other noble metals or their oxides, or carbon filmcompositions. The preferred material for the base layer 20 is goldcoated polyester film.

Reagent holding layer 30 has a first electrode opening 32 which exposesa portion of first conductive path 22, a second electrode opening 34which exposes a portion of second conductive path 24, and a thirdelectrode opening 36 which exposes a portion of third conductive path26. Reagent holding layer 30 is made of a plastic material, preferably amedical grade, one-sided adhesive tape available from Adhesive Research,Inc., of Glen Rock, Pa. or Global Instrument Corporation (GIC) (Taiwan).Acceptable thicknesses of the tape for use in the present invention arein the range of about 0.001 in. (0.025 mm) to about 0.005 in. (0.13 mm).The preferred thickness is about 0.003 in. (0.075 mm). It should beunderstood that the use of a tape is not required. Reagent holding layer30 may be made from a plastic sheet and may be coated with a pressuresensitive adhesive, a photopolymer, ultrasonically-bonded to base layer20, or silk-screened onto the base layer 20 to achieve the same resultsas using the polyester tape mentioned.

The three electrode openings 32, 34 and 36 define electrode wells W, Rand S, respectively, and hold chemical reagents forming a workingelectrode, a reference electrode, and a substrate electrode. Preferably,electrode well W is loaded with a chemically oxidizing reagent and atleast a polymer binder. Substrate electrode well S is loaded with asimilar chemistry to electrode well W, but with the addition of achemical agent that consumes bilirubin before the bilirubin can beoxidized by the mediator at the surface of the electrode by a voltagethat is applied to the bilirubin sensor during use. The term “consumingbilirubin” means that the bilirubin is removed by the oxidase so that itcannot be oxidized by the mediator when voltage is applied to theelectrode. Preferably, the chemical agent is a bilirubin sensitiveenzyme and, more preferably, it is bilirubin oxidase. One or morechemical components such as additional polymers, stabilizers, andbulking agents may be optionally included in the reagent matrix. Areference reagent matrix is loaded in electrode well R.

Preferably, the reference matrix contains at least a chemicallyoxidizing reagent such as a reduced form of a redox mediator, anoxidized form of redox mediator, or a mixture of a reduced and anoxidized form of a redox mediator. For example, potassium ferricyanide,or potassium ferrocyanide, or a mixture of potassium ferricyanide andpotassium ferrocyanide may be loaded to make the reference electrodefunction when using the preferred conductive coating material. Themixture of potassium ferricyanide and potassium ferrocyanide may beprepared such that the potassium ferricyanide concentration is in therange of up to about 10%, while the potassium ferrocyanide concentrationis in the range of up to about 5%. In the alternative, the referenceelectrode (electrode well R) may be loaded with an Ag/AgCl layer (e.g.,by applying Ag/AgCl ink or by sputter-coating (a) a Ag layer followed bychloridizing the Ag or (b) a Ag/AgCl layer) or other reference electrodematerials that do not require a redox mediator to function properly.

The size of the reagent holding openings is preferred to be made assmall as possible in order to make the sample chamber of the bilirubinsensor as short as possible in order to minimize the volume of samplerequired for each test measurement. The three reagent holding/electrodeopenings 32, 34 and 36 are aligned with each other and are spaced about0.02 in. (0.508 mm) to about 0.05 in. (1.27 mm) from each other. Thecircular reagent holding openings are for illustrative purposes only. Itshould be understood that the shape of the reagent holding openings andthe distance between each of the electrodes is not critical. In fact,the reagent holding openings may differ in surface area from each otherso long as the ratio of the surface areas remain substantially constantfrom one sensor to another.

The positional arrangement of the working, substrate and referenceelectrodes in the channel is not critical for obtaining usable resultsfrom the bilirubin sensor. The possible electrode arrangements withinthe sample fluid channel may be W-S-R, W-R-S, R-W-S, S-W-R, S-R-W, orR-S-W, with the arrangement listed as the electrodes would appear fromthe sample inlet 18 of laminated body 12 to the vent opening 52. Thepreferred position was found to be W-R-S; that is, as the fluid sampleenters sampling end 14 of laminated body 12, the fluid sample wouldcover the working electrode first, then the reference electrode, andthen the substrate electrode.

The three electrodes are each in electric contact with separateconductive paths. The separate conductive paths terminate and areexposed for making an electrical connection to a reading device on theend opposite from the sample inlet 18 of laminated body 12.

Channel forming layer 40 has a channel notch 42 located at the fluidsampling end 14. The length of channel notch 42 is such that whenchannel forming layer 40 is laminated to reagent holding layer 30,electrode areas W, R and S are within the space defined by channel notch42. The length, width and thickness of the channel notch 42 define thecapillary chamber volume.

Channel forming layer 40 is made of a plastic material, preferably amedical grade, double-sided pressure-sensitive adhesive tape availablefrom Adhesive Research, Inc., of Glen Rock, Pa. or Global InstrumentCorporation (Taiwan). The thickness of the tape is preferably in therange of about 0.001 in. (0.025 mm) to about 0.010 in. (0.25 mm).Channel notch 42 can be made with a laser or by die-cutting (thepreferred method). The length of channel notch 42 is about 0.200 in.(5.08 mm) to about 0.250 in. (6.35 mm), the width is about 0.05 in.(1.27 mm) to about 0.07 in. (1.778 mm) and the thickness is about 0.0035(0.089 mm) to about 0.0065 in. (0.165 mm). It should be understood thatthe thickness and the size of channel notch 42 are not critical.

Cover 50, which is laminated to channel forming layer 40, has ventopening 52 spaced from fluid sampling end 14 of bilirubin sensor 10 toensure that the sample in the sample chamber 17 will completely coverelectrode areas W, R and S. Vent opening 52 is positioned in cover 50 sothat it will expose a portion of and partially overlay channel notch 42at or near the closed end of the channel notch 42. Vent opening 52 maybe any shape but is illustrated as a rectangle having dimensions ofabout 0.08 in. (2 mm) by about 0.035 in. (0.9 mm). The preferredmaterial for cover 50 is a polyester film. In order to facilitatecapillary action of the sample fluid within the sample chamber, it isdesirable for the polyester film to have a highly hydrophilic surface onthat portion of the polyester film that forms the capillary chamber.Transparency films from 3M or from GIC can be used. Cover 50 mayoptionally include an inlet notch 54 to prevent an inadvertent occlusionof sample inlet 18 (which can prevent the proper transfer of the samplefluid to the electrodes) when applying a blood sample to sample chamber17.

FIG. 3 illustrates a 3-layer bilirubin sensor 10′. Like the 4-layerembodiment, bilirubin sensor 10′ has a laminated body 12, a fluidsampling end 14, an electrical contact end 16, and a vent opening 52.Fluid sampling end 14 includes a sample chamber 17 between a sampleinlet 18 and vent opening 52. Electrical contact end 16 has threediscrete conductive contacts 16 a, 16 b and 16 c.

As can be seen from FIG. 4, laminated body 12 includes a base layer 20,a channel forming layer 40, and a cover 50. As noted earlier, all layersof laminated body 12 are made of a dielectric material, preferablyplastic. Unlike the 4-layer embodiment, there is no separate reagentholding layer in the 3-layer embodiment. Channel forming layer 40 alsodelineates the area in which a pre-determined amount of reagent mixturesare disposed onto the conductive paths as three distinct drops ordroplets on the working, substrate and reference electrodes,respectively.

Turning now to FIG. 5, there is illustrated another embodiment of thepresent invention showing a bilirubin sensor 500. Bilirubin sensor 500has a laminated body 512, a sample receiving well 514 and an electricalcontact end 516. Laminated body 512 has a base layer 520 and a cover550. Cover 550 has a sample opening 552 that forms, when combined withbase layer 520, sample receiving well 514. Base layer 520 has at leastthree electrical paths 522, 524 and 526, which have a first portionexposed at electrical contact end 516 for connection to a meter device(not shown) and a second portion exposed by sample receiving well 514.

The second portion of electrical paths 522, 524 and 526 exposed bysample receiving well 514 create at least a working electrode W, asubstrate electrode S and at least a reference/counter electrode R. Afirst reagent mixture 560 contains at least a polymer binder and achemically oxidizing reagent and is disposed on the working electrode W.A second reagent mixture 562 contains a polymer binder, a chemicallyoxidizing reagent and a chemical agent that consumes bilirubin and isdisposed on the substrate electrode S. The reference/counter electrode Rmay contain any reference material previously disclosed. In thisembodiment of the present invention, sample receiving well 514 serves asboth the sample inlet and the sample chamber for receiving a fluidsample such as blood for the determination of bilirubin.

It should be understood that the conduit paths in any of the embodimentsdisclosed herein may be made from any non-corroding metal. Carbondeposits such as for example carbon paste or carbon ink may also be usedas the conduit paths, all as is well known by those of ordinary skill inthe art.

Chemical Reagents

Enzyme

The bilirubin sensor of the present invention includes at least achemical agent in the substrate electrode S that consumes bilirubin. Thechemical agent is preferably a bilirubin sensitive enzyme, and, morepreferably, bilirubin oxidase, which can selectively react withbilirubin in the sample fluid. The substrate electrode is essential inthe present invention for making an accurate determination of theconcentration of bilirubin in a body fluid. As previously explained, theuse of such a substrate electrode helps to distinguish the oxidationcurrent caused by bilirubin from the oxidation current caused by otheroxidizable species in the sample fluid. A commercially availablebilirubin oxidase from Sigma Chemical Company, St. Louis, Mo., or fromAmono, Japan is used for the preparation of the bilirubin substrateelectrode. The concentration of bilirubin oxidase in the reagent mixtureis in the range of about 10 units/ml to 90 units/ml.

Chemically Oxidizing Reagents

Chemically oxidizing reagents such as redox mediators may optionally beincluded in the bilirubin sensor. It is preferable to use a redoxmediator in its oxidized form. It is also desirable that the reducedform of the mediator is capable of being oxidized electrochemically atthe electrode surface at the applied potential. It is further desirablethat the mediator is stable in the reagent matrix. When used in areference electrode, it is still further desirable that the mediator canmake the reference electrode function properly. The redox mediator canbe selected from, but not limited to, various metal compounds andorganic redox compounds. Examples of acceptable redox mediators includepotassium (or sodium) ferricyanide, ferrocene and its derivatives,cupric compounds, nitrite compounds, ruthenium compounds such ashexaammineruthenium(III) chloride and its derivatives, and osmiumcomplexes. The preferred mediator is potassium ferricyanide. Theconcentration of potassium ferricyanide in the reagent mixture ispreferably about 1% to about 15% (w/w) of the reagent mixture.

Because the use of a chemically oxidizing reagent is optional, the redoxmediator is not a necessary ingredient in the reagent matrix to bedisposed in the working electrode and substrate electrode wells of thebilirubin sensor. In this embodiment, the working electrode is onlyloaded with a polymer binder and, preferably, with a bulking reagent, asurfactant and a buffer. The substrate electrode is loaded with the samechemicals as the working electrode but with the addition of a chemicalagent that consumes bilirubin. The preferred chemical agent is bilirubinoxidase. A higher positive potential (e.g., 0.8 V) is needed to beapplied between the working/substrate electrodes and the referenceelectrode when no chemically oxidizing reagent is included. All of thereductants including bilirubin in the sample will contribute theoxidation current response signal when the higher positive potential isapplied at the working electrode. Meanwhile, at the substrate electrode,bilirubin will be consumed by the bilirubin oxidase. Consequently, theoxidation current response will be from all of the reductants in thesample fluid except bilirubin. The differential current response betweenthe working electrode and the substrate electrode is related to theconcentration of bilirubin.

Polymers

The polymer used as the binder in the reagent matrix should besufficiently water-soluble and should also be capable of stabilizing andbinding all other chemicals in the reagent to the conductive surfacelayer in the electrode area. Suitable polymers include, but are notlimited to, polyethylene oxide (PEO), polyvinyl pyrolidone, starch,methylcellulose, polyvinyl alcohol (PVA), carboxy methyl cellulose(CMC), and polyamino acids. The reagent binder may be a single polymeror a combination of polymers. The preferred binder in the presentinvention is a combination of polyethylene oxide (PEO) andmethylcellulose. PEO's molecular weight ranges from thousands tomillions and is available from Scientific Polymer Products, NY, USA. Theconcentration of PEO in the reagent mixture is preferably about 0.04%(W/W) to about 2%. Methylcellulose, which is available under the brandname of Methocel 60 HG (Cat. No. 64655, Fluka Chemicals, Milwaukee,Wis., USA) has a concentration in the reagent mixture preferably in therange of about 0.02% (w/w) to about 5%.

Surfactants

The surfactant may be optionally included in the reagent mixture tofacilitate dispensing of the reagent mixture into the electrode areas.The surfactant also helps in quickly dissolving the dry chemicalreagents when a sample fluid enters the sample channel of the bilirubinsensor. The amount and type of surfactant is selected to assure thepreviously mentioned functions and to avoid a denaturing effect on theenzymes. Surfactants can be selected from, but are not limited to,various anionic, cationic, non-ionic, and zwitterionic detergents.Examples of acceptable surfactants are polyoxyethylene ether, Tween 20,sodium cholate hydrate, hexadecylpyridinium cholide monohydrate andCHAPs. The preferred surfactant is a polyoxyethylene ether. Morepreferably, it is t-octylphenoxypolyethoxyethanol and is available underthe brand name Triton X-100. The concentration of Triton X-100 in thereagent mixture is preferably about 0.01% (w/w) to about 2%.

Buffer

Optionally, a buffer may be present along with a redox mediator in driedform in the sensor strip of the present invention. The buffer is presentin a sufficient amount so as to substantially maintain the pH of thereagent mixtures. Examples of suitable buffers include citric acid,phosphates, Tris, and the like. In the present invention, the pH of thebuffer is preferably in the range from about 5.0 to about 8.5.

Bulking Reagent

An optional bulking agent that is water soluble and an inactiveingredient is preferably added into the reagent mixture. The use of abulking agent is advantageous when an electrode forming layer is used tocontain the reagent matrix such that the electrode openings in theelectrode forming layer will not trap bubbles when a sample fluid fillsthe capillary channel. Various sugars such as, for example, trehalose,galactose, glucose, sucrose, lactose, mannitol, mannose, fructose,sucrose, lactose, lactitol, sorbitol, xylitol, nicotinamide, maltose,and the like, can be added into the reagent mixture as long as they donot react with other ingredients and are inactive at the electrodesurface. The bulking agent can be one chemical or a combination ofchemicals. The amount of bulking agent in the reagent mixture is in therange from about 1% to about 15% (w/w).

Accordingly, the preferred reagent mixture (referred to below as“reagent mixture 1”) used for the bilirubin working electrode (W) andreference electrode (R) contains Methocel 60 HG, polyethylene oxide,potassium ferricyanide, Triton X-100, bulking agents, and Tris buffer(pH 8). The preferred reagent mixture (referred to below as “reagentmixture 2”) used for the substrate electrode (S) contains Methocel 60HG, polyethylene oxide, potassium ferricyanide, Triton X-100, bulkingagents, bilirubin oxidase, and Tris buffer (pH 8).

Preparation of the Reagent Mixtures

Reagent mixture 1 is preferably prepared in two steps, although it canbe prepared in one step:

Step 1: Into Tris buffer (pH 8), add Methocel 60 HG, polyethylene oxide,methylcellulose, bulking agent, and Triton X-100. Stir the solutionuntil dissolved using a magnetic stirrer.

Step 2: Into the above solution, add potassium ferricyanide. Stir thesolution until dissolved. The resulting solution is ready fordispensing.

Reagent mixture 2 is prepared in two steps:

Step 1: Into Tris buffer (pH 8), add Methocel 60 HG, polyethylene oxide,methylcellulose, bulking agent, and Triton X-100. Stir the solutionuntil dissolved using a magnetic stirrer.

Step 2: Into the above solution, add potassium ferricyanide andbilirubin oxidase. Stir the solution until dissolved. The resultingsolution is ready for dispensing.

Sensor Construction

Assembly of the various embodiments of the present invention isrelatively straightforward. Generally for the 4-layer configuration, thebase layer and reagent holding layer are laminated to each otherfollowed by dispensing the appropriate reagent mixture into each of thereagent holding openings. After drying the reagent mixture, the channelforming layer is laminated onto the reagent holding layer and the coveris then laminated onto the channel forming layer. For the 3-layerconstruction, the base layer and the channel forming layer are laminatedto each other followed by dispensing the appropriate reagent mixture asdistinct drops/droplets into the U-shaped channel onto their respectiveconductive surface areas. After drying the reagent mixture, the cover isthen laminated onto the channel forming layer.

More particularly, a piece of a gold polyester film is cut to shape asillustrated in FIG. 2, forming base layer 20 of sensor 10. A laser(previously disclosed) is used to score the gold polyester film. Asillustrated in FIG. 2, the film is scored by the laser such that threeelectrodes at sample fluid end 14 and three contact points 22, 24 and 26are formed at electrical contact end 16. The scoring line is very thinbut sufficient to create three separate electrical paths. A scoring line28 may optionally be made, but is not necessary, along the outer edge ofbase layer 20 to avoid potential static problems which could cause anoisy signal from the finished sensor 10.

A piece of one-sided adhesive tape is then cut to size and shape,forming reagent holding layer 30 so that it will cover a major portionof conductive layer 21 of base layer 20 except for exposing a smallelectrical contact area illustrated in FIG. 1 by reference number 16.

Before attaching reagent holding layer 30 to base layer 20, threecircular openings 32, 34 and 36 of substantially equal size are punchedby laser, or by mechanical means such as a die-punch assembly, creatingelectrode openings 32, 34 and 36 in reagent holding layer 30. Thepreferred hole size for openings 32, 34 and 36 has a typical diameter ofabout 0.030 in. (0.76 mm). As illustrated in FIG. 2, electrode openings32, 34 and 36 are aligned with each other and have a spacing of about0.020 in. (0.508 mm) to about 0.050 in. (1.27 mm) between adjacentopenings. The circular openings are for illustrative purposes only. Itshould be understood that the shape and size of the openings is notcritical. The circular openings do not have to be substantially equal insize so long as the ratio of the surface areas remains substantiallyconstant. As stated previously, the preferred arrangement of theelectrodes formed in openings 32, 34 and 36 is W (working electrode), R(reference electrode) and S (substrate electrode). Reagent holding layer30 is then attached to base layer 20 in such a way as to define theelectrode wells W, R and S.

Approximately 0.05 to 0.09 microliters (μL) of reagent mixture 1 isdispensed into the working and reference electrode areas W and R. Asdescribed above, reagent mixture 1 is preferably a mixture of a bulkingagent, a polymer binder, a surfactant, a redox mediator and a buffer.Similarly, approximately 0.05 to 0.09 μL of reagent mixture 2 isdispersed into substrate electrode area S.

After dispensing the reagents in their respective electrode areas, thereagents are dried at a temperature in the range of about roomtemperature to about 60° C. The length of time required to dry thereagents is dependent on the temperature at which the drying process isperformed.

After drying, a piece of double-sided tape available from AdhesiveResearch or GIC is fashioned into chamber forming layer 40 containingU-shaped channel 42. Chamber forming layer 40 is then layered ontoreagent holding layer 30. As mentioned earlier, this chamber forminglayer 40 serves as a spacer and defines the size of sample chamber 17.

A piece of a transparency film available from 3M or from GIC isfashioned into top layer 50. A vent opening 52 is made using the laserpreviously mentioned or by means of a die-punch. Vent opening 52 islocated approximately 0.180 in. (4.57 mm) from fluid entrance 54. Toplayer 50 is aligned and layered onto chamber forming layer 40 tocomplete the assembly of bilirubin sensor 10, as illustrated in FIG. 1.

Testing the Bilirubin Sensor

When a fluid sample is applied to a single strip of the presentinvention, the fluid sample enters the channel through the samplinginlet 18 and flows over W, R and S and stops at the threshold of thevent opening 52.

Chronoamperometry (i-t curve) was used for measurement of the currentresponse of the bilirubin sensor using an Electrochemical Analyzer(Model 812, CH Instruments, Austin, Tex., USA). Once a blood sampleenters the sensor, a potential of 0.3 to 0.5 volts is applied across theworking/substrate electrodes and the reference electrode on thosesensors that use a chemically oxidizing reagent in the reagent matrix.For sensors constructed with a reagent matrix that does not contain achemically oxidizing reagent (e.g., redox mediator), a potential ofabout 0.8 volts is used. The bilirubin concentration of the same bloodsample is measured with a Dade Behring Dimension® RxL Clinical ChemistrySystem.

The above described embodiments are based on amperometric analyses.Those skilled in the art, however, will recognize that a sensor of theinvention may also utilize coulometric, potentiometric, voltammetric,and other electrochemical techniques to determine the concentration ofan analyte in a sample.

The following examples illustrate the unique features of the presentinvention.

EXAMPLE 1 Determination of Bilirubin Concentration Using a ChemicalMatrix Containing Ferricyanide and Bilirubin Oxidase

Blood samples with different bilirubin concentrations were tested withthe bilirubin sensors of the present invention using the above-describedreagent formulations. Table 1 shows the corrected current response innanoamps in a blood sample with varying levels of bilirubin. Thecorrected current response means the differential current responsebetween the working electrode and the substrate electrode.

TABLE 1 Linearity Bilirubin Corrected current concentration (mg/dL)response (nA) 0.5 32.9 1.7 34.6 3.1 37.3 4.5 38.8 7.0 43.8 10.5 48.413.5 54.2 20.6 66.0 25.0 71.9

A graphical illustration of the test data is shown in FIG. 6. The testresults demonstrate that the sensor of the present invention has alinear response (current response vs. bilirubin concentration) over abilirubin concentration range from 0 to 25 mg/dL.

EXAMPLE 2 Precision of Bilirubin Sensor

The precision of the bilirubin sensor of present invention wasinvestigated with two levels of bilirubin (2.3 mg/dL and 8.8 mg/dL) inblood samples. Table 2 shows the precision of the response of thebilirubin sensor of the present invention. A total of 10 measurementswere made on each level of bilirubin blood sample.

TABLE 2 Precision Results Measurement No mg/dL mg/dL 1 2.2 9.1 2 2.1 8.83 2 9.2 4 2.5 9 5 2.5 9.2 6 2.3 8.9 7 2.2 9.1 8 2 8.8 9 2.2 9.3 10  2.29 Mean 2.22 9.04 CV (%) 7.9 1.9

A graphical illustration of the test data is shown in FIG. 7.

EXAMPLE 3 Determination of Bilirubin Concentration Using a ChemicalMatrix Containing Hexaammineruthenium (III) Chloride and BilirubinOxidase

Blood samples with different bilirubin concentrations were tested withthe bilirubin sensors of the present invention with above reagentformulations using a different chemically oxidizing reagent. Table 3shows the current response in nanoamps in a blood sample with varyinglevels of bilirubin.

TABLE 3 Linearity Bilirubin Current concentration (mg/dL) response (nA)0.7 12.5 2.3 14.0 4.0 16.3 5.5 18.6 8.8 23.5 13.5 29.2 15.8 31.9 23.044.9

A graphical illustration of the test data is shown in FIG. 8. The testresults demonstrate that the sensor of the present invention has alinear response (current response vs. bilirubin concentration) over abilirubin concentration range from 0 to 23 mg/dL.

EXAMPLE 4 Determination of Bilirubin Concentration Using a ChemicalMatrix that Does Not Contain a Chemically Oxidizing Reagent

Blood sample with different bilirubin concentrations were tested withthe bilirubin sensors of the present invention with above reagentformulations. Table 4 shows the current response in nanoamps in a bloodsample with varying levels of bilirubin.

TABLE 4 Linearity Bilirubin Current concentration (mg/dL) response (nA)1.0 8.7 4.5 11.5 9.7 13.5 16.3 16.7 21.6 21.3 28.3 27.1

A graphical illustration of the test data is shown in FIG. 8. The testresults demonstrate that the sensor of the present invention has alinear response (current response vs. bilirubin concentration) over abilirubin concentration range from 0 to 28 mg/dL.

Although the preferred embodiments of the present invention have beendescribed herein, the above description is merely illustrative. Furthermodification of the invention herein disclosed will occur to thoseskilled in the respective arts and all such modifications are deemed tobe within the scope of the invention as defined by the appended claims.

1. A bilirubin sensor comprising: a working electrode with a firstchemical matrix disposed thereon, the first chemical matrix containing abinder, wherein the first chemical matrix is capable of oxidizing alloxidizable species in a fluid sample; a substrate electrode with asecond chemical matrix disposed thereon, the second chemical matrixcontaining a binder and a chemical agent that consumes bilirubin,wherein the second chemical matrix is capable of oxidizing alloxidizable species in the fluid sample except bilirubin; a referenceelectrode; a sample chamber containing the working electrode, thesubstrate electrode and the reference electrode; means for applying apotential between the reference electrode and the working electrode toobtain a first current; means for applying a potential between thereference electrode and the substrate electrode to obtain a secondcurrent; means for correlating the first current to a concentration ofall oxidizable species in the fluid sample and for correlating thesecond current to a concentration of all oxidizable species in the fluidsample except bilirubin; means for subtracting the second current fromthe first current to obtain a current difference; and means forcorrelating the current difference to a concentration of bilirubin inthe fluid sample.
 2. The bilirubin sensor of claim 1 wherein thechemical agent is an oxidoreductase of bilirubin.
 3. The bilirubinsensor of claim 2 wherein the oxidoreductase of bilirubin is bilirubinoxidase.
 4. The bilirubin sensor of claim 1 wherein the first chemicalmatrix and the second chemical matrix contains a chemically oxidizingreagent.
 5. The bilirubin sensor of claim 4 wherein said chemicallyoxidizing reagent is a redox mediator.
 6. The bilirubin sensor of claim4 wherein the chemically oxidizing reagent is selected from the groupconsisting of potassium ferricyanide, sodium ferricyanide, ferrocene,ferrocene derivatives, cupric sulfate, hexaamineruthenium (III)chloride, hexaamineruthenium (III) chloride derivatives, and osmiumcomplexes.
 7. The bilirubin sensor of claim 1 wherein the bilirubinsensor is an amperometry-based sensor.
 8. The bilirubin sensor of claim1 wherein the first chemical matrix and the second chemical matrixincludes one or more reagents selected from the group consisting of abulking agent, a surfactant and a buffer.
 9. The bilirubin sensor ofclaim 8 wherein the buffer has a pH in the range from about 5.0 to about8.5.
 10. The bilirubin sensor of claim 1 wherein the binder includes oneor more chemical reagents selected from the group consisting ofpolyethylene oxide, polyvinyl pyrolidone, starch, methylcellulose,polyvinyl alcohol, carboxy methyl cellulose, and polyamino acids.
 11. Abilirubin sensor comprising: a working electrode with a first chemicalmatrix disposed thereon, the first chemical matrix containing a wholeblood filtering component, wherein the first chemical matrix is capableof oxidizing all oxidizable species in a fluid sample; a substrateelectrode with a second chemical matrix disposed thereon, the secondchemical matrix containing a whole blood filtering component and achemical agent that consumes bilirubin, wherein the second chemicalmatrix is capable of oxidizing all oxidizable species in the fluidsample except bilirubin; a reference electrode; a sample chambercontaining the working electrode, the substrate electrode and thereference electrode; means for applying a potential between thereference electrode and the working electrode to obtain a first current;means for applying a potential between the reference electrode and thesubstrate electrode to obtain a second current; means for correlatingthe first current to a concentration of all oxidizable species in thefluid sample and for correlating the second current to a concentrationof all oxidizable species in the fluid sample except bilirubin; meansfor subtracting the second current from the first current to obtain acurrent difference; and means for correlating the current difference toa concentration of bilirubin in the fluid sample.
 12. The bilirubinsensor of claim 11 wherein the chemical agent is an oxidoreductase ofbilirubin.
 13. The bilirubin sensor of claim 12 wherein theoxidoreductase of bilirubin is bilirubin oxidase.
 14. The bilirubinsensor of claim 11 wherein the first chemical matrix and the secondchemical matrix contains a chemically oxidizing reagent.
 15. Thebilirubin sensor of claim 14 wherein said chemically oxidizing reagentis a redox mediator.
 16. The bilirubin sensor of claim 14 wherein thechemically oxidizing reagent is selected from the group consisting ofpotassium ferricyanide, sodium ferricyanide, ferrocene, ferrocenederivatives, cupric sulfate, hexaamineruthenium (III) chloride,hexaamineruthenium (III) chloride derivatives, and osmium complexes. 17.The bilirubin sensor of claim 11 wherein the bilirubin sensor is anamperometry-based sensor.
 18. The bilirubin sensor of claim 11 whereinthe first chemical matrix and the second chemical matrix includes one ormore reagents selected from the group consisting of a bulking agent, asurfactant and a buffer.
 19. The bilirubin sensor of claim 18 whereinthe buffer has a pH in the range from about 5.0 to about 8.5.
 20. Amethod of measuring bilirubin in a fluid sample comprising the steps of:disposing the fluid sample into a sample chamber of a bilirubin sensor;applying a potential between a reference electrode and a workingelectrode in the sample chamber, wherein the working electrode has afirst chemical matrix disposed thereon that contains a binder and thatserves to oxidize all oxidizable species in the fluid sample; measuringa first current generated between the working electrode and thereference electrode, wherein the first current is correlated to aconcentration of all oxidizable species in the fluid sample; applying apotential between the reference electrode and a substrate electrode inthe sample chamber, wherein the substrate electrode has a secondchemical matrix disposed thereon that contains a binder and a chemicalagent that consumes bilirubin so as to oxidize all oxidizable species inthe fluid sample except bilirubin; measuring a second current generatedbetween the reference electrode and the substrate electrode, wherein thesecond current is correlated to a concentration of all oxidizablespecies in the fluid sample except bilirubin; subtracting the secondcurrent from the first current and obtaining a current difference; andcorrelating the current difference to a concentration of bilirubin inthe fluid sample.
 21. The method of claim 20 further comprising:measuring a resistance value of the fluid sample between the referenceelectrode and one of the working electrode, the substrate electrode, orboth; applying the resistance value to a first equation and determiningthe hematocrit level of the fluid sample from the first equation; andcalculating a corrected concentration of bilirubin using a secondequation to correct for the presence of hematocrit in the fluid sample.22. The method of claim 20 wherein the potential applying steps includeapplying a potential of about 0.800 volts.
 23. The method of claim 20wherein the potential applying steps include applying a potential ofabout 0.350 volts when the first chemical matrix and the second chemicalmatrix contains a chemically oxidizing reagent.
 24. The method of claim20 further comprising triggering the current measuring step when thefluid sample contacts the working electrode, the substrate electrode andthe reference electrode.
 25. The method of claim 20 further comprising:measuring a resistance value of the fluid sample between the referenceelectrode and one of the working electrode, the substrate electrode, orboth; and applying the resistance value to a derived algorithm tocorrect for the presence of hematocrit in the fluid sample.